Page 177 - Geothermal Energy Systems Exploration, Development, and Utilization

3.8 Risks 153
costs due to damage or loss of downhole tools (e.g., DHM/MWD approximately
¤500 000); sometimes to partial or (worst case) total loss of the hole (sidetrack/new
hole); seldom environmental impact; all risks are influencing costs.
Occurrence of risks cannot be totally avoided. Therefore it is highly recommended
to calculate project budget on realistic assumptions and to take care for an additional
part of funding (risk margin).

3.8.1
Evaluating Risks

3.8.1.1 Poor or Wrong Geological Profile Forecast
The predrill geological information can be poor if not all available geodata for a
geothermal field are compiled together, for example, in a geological model. How-
ever, the data quantity and quality for an explorated field might be insufficient or
limited. If preexisting data are used, a reprocessing and subsequent 3D structural
modeling, integrating the knowledge of regional geology, petrophysics, and geo-
physics, will provide the best picture of subsurface structure. Well path trajectory
can be directly planned within the geological models and can be updated on-site
during operation supporting real-time surveillance of drilling progress. Finally,
3D geological models help also to specify uncertainties and evaluating risk. If
geological interpretations are wrong or if a compilation of geological knowledge is
poor, a lot of problems may arise as
• possibly wrong casing setting depth
– too less strings, drilling problems;
– target not or with insufficient diameter reachable;
• fault zones and/or targets not where expected
– drilling problems;
– ‘‘dry’’ hole.

3.8.1.2 Poor Well Design
Poor well design is a typical planning risk. It can happen due to a wrong geological
profile forecast as described above, but also other reasons can lead to it as shown
below.
• Wrong casing setting depth: Having chosen the wrong casing setting depth for a
well (e.g., setting a casing string too high) may cause drilling problems and may
ask for running the next casing size earlier than planned. Consequently too less
casing strings may available for the well. This may cause that the target cannot be
reached or only with insufficient diameter, causing problems (and higher costs)
during production and/or injection.
• Insufficient casing design: An insufficient casing design (e.g., wall thickness too
small or material strength (grade) not adequately chosen) can cause
– Casing collapse: This hazard occurs specially in deep evaporitic environments if
cementationis incomplete. Two mechanisms are recognized for casing collapse
in salt rock: (i) sectorial pressurization and (ii) shear by slipping (Smart, Ford,